1. Field of the Invention
The present invention generally relates to a laser system for generating a short-pulse high-power laser beam, and specifically to a driver laser system for applying a laser beam to a target in an LPP (laser produced plasma) type EUV (extreme ultra violet) light source apparatus for generating extreme ultra violet light to be used for exposure of semiconductor wafers or the like.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 45 nm, further, microfabrication of 32 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 32 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages in that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be explained. By applying a laser beam to a target material supplied into a vacuum chamber, the target material is excited and turned into plasma. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror for selectively reflecting a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit. For the purpose, on a reflecting surface of the EUV collector mirror, a multilayer film with alternately stacked molybdenum (Mo) and silicon (Si) (Mo/Si multilayer film) is formed.
As an EUV light source that can generate EUV light with high efficiency, an LPP type EUV light source apparatus for applying a laser beam generated by a CO2 laser to a tin (Sn) target has been proposed, and technology developments are centered on the apparatus. At present, the required output of the EUV light source apparatus in the immediate future is 140 W, and even in the LPP type EUV light source apparatus with a combination of the CO2 laser and the tin target, the output as EUV light that can be used for exposure is about 1% to 4% of the laser output. Accordingly, in the case of the LPP type EUV light source apparatus having an output of 140 W, the laser output of about 10 kW or more, for example, is necessary. Further, the case where the sensitivity of the resist necessary for exposure becomes lower is envisioned, and the output of the EUV light source apparatus of about 300 W may be necessary. In this case, the laser output of 20 kW to 30 kW may be necessary.
For industrial use, a CO2 laser having an output of 20 kW is commercially available, and this is a CW (continuous wave) laser for continuously outputting a laser beam. On the other hand, for EUV generation, a short-pulse CO2 laser for outputting a pulse laser beam having a pulse width of about 10 ns to 100 ns is necessary. A high-power short-pulse CO2 laser is not commercially available, and thus, a high-power driver laser system is realized by using a low-power short-pulse CO2 laser as a laser oscillator and amplifying the output of the laser oscillator with plural amplifiers.
As an example of a CO2 laser system for outputting high pulse energy, V. A. Adamovich et al., “TIR-1 carbon dioxide laser system for fusion”, Optical Society of America, 1980, pp. 313-318, Reprinted from Applied Optics, Vol. 19(6), 918-923 (March, 1980) discloses a CO2 laser system for fusion. The laser oscillator is a TEA CO2 laser and generates a pulse laser beam by high-voltage pulse discharge. The pulse laser beam is amplified by multiple TEA CO2 laser amplifiers by synchronizing discharge of the TEA CO2 laser amplifiers with the pulse laser beam.
Further, as a related technology, U.S. Pat. No. 7,439,530 B2 (page 6, FIG. 8) discloses a schematic diagram showing a configuration of a driver laser system of an EUV light source apparatus as shown in FIG. 20. As shown in FIG. 20, the driver laser system 200 includes a laser oscillator (OSC) 210 for generating a short-pulse laser beam and three CO2 laser amplifiers (AMP) 221-223 for amplifying the laser beam generated by the laser oscillator 210. As disclosed, the higher power is obtained by amplifying the pulse laser beam outputted from the laser oscillator 210 with the multiple laser amplifiers 221-223. The amplified pulse laser beam 6 outputted from the laser amplifier 223 is collected by a collective optics 2 on the target to generate EUV light 8.